Observations of Freshening in the Northwest Pacific Subtropical Gyre near Luzon Strait

Abstract

Argo observations reveal that the salinity in the North Pacific subtropical gyre near Luzon Strait gradually declined by 0.2 (practical salinity scale used) from 2003 to 2007 over a depth range of 100 m to 200 m. Such freshening is also found in the outputs of the Estimating the Circulation and Climate of the Ocean (ECCO) model. The possible mechanisms for the freshening are investigated using the surface freshwater flux (E-P) data, the ECCO outputs and a salt budget equation for the upper ocean. Our analysis indicates that the magnitude of the salinity change caused by the surface freshwater flux anomaly is far smaller than observed, suggesting that the surface freshwater flux anomaly is not sufficient to account for the observed freshening. In fact, the salinity anomaly is closely linked to a pronounced freshening at the northeast corner of the study area from 2003 to 2007. The advection of salinity anomalies in the western North Pacific Ocean between 25°N and 35°N via a southwestward flow in the "C-shaped" region associated with the Kuroshio system is the main mechanism responsible for the observed freshening in the study area.RÉSUMÉ [Traduit par la rédaction] Les observations Argo révèlent que la salinité dans le gyre subtropical du Pacifique Nord près du détroit de Luçon a graduellement diminué de 0,2 (en utilisant l'échelle de salinité pratique) entre 2003 et 2007 dans un intervalle de profondeur de 100 à 200 m. Un adoucissement comparable s'observe aussi dans les sorties du modèle ECCO (Estimating the Circulation and Climate of the Ocean). Nous examinons les mécanismes pouvant expliquer l'adoucissement au moyen des données de flux d'eau douce en surface (E-P), des sorties de l'ECCO et de l'équation du bilan du sel pour la couche supérieure de l'océan. Notre analyse indique que l'ampleur du changement de salinité causé par l'anomalie de flux d'eau douce en surface est beaucoup plus petite que ce qui est observé, ce qui suggère que l'anomalie de flux d'eau douce en surface ne suffit pas à expliquer l'adoucissement observé. En fait, l'anomalie de salinité est étroitement liée à un adoucissement prononcé dans le coin nord-est de la zone à l'étude de 2003 à 2007. L'advection des anomalies de salinité dans l'ouest du Pacifique Nord entre 25°N et 35°N par un écoulement vers le sud-ouest dans la région en forme de « C » associée au système du Kuroshio est le principal mécanisme responsable de l'adoucissement observé dans la zone à l'étude.

Propagation of the subsurface freshening water and its major source in the northwestern Pacific

“…The possible connection between the subtropics and the western boundary via the subsurface salinity propagation in the North Pacific is an important issue and has been recently examined [e.g., Sasaki et al, 2010;Ren and Riser, 2010;Li et al, 2012;Yan et al, 2012Yan et al, , 2013Nagano et al, 2014;Nan et al, 2015]. Some studies suggest that the anomalies originating in the northeastern subtropical Pacific can subduct and penetrate to the subsurface of the northwestern Pacific [e.g., Sasaki et al, 2010;Ren and Riser, 2010;Li et al, 2012;Kolodziejczyk and Gaillard, 2012], but some recent studies, on the other hand, demonstrate that the subsurface salinity anomalies in the northwestern Pacific cannot be traced back to those of the eastern subtropics [Yan et al, 2012[Yan et al, , 2013Nagano et al, 2014;Nan et al, 2015]. Here our study shows the subsurface salinity anomalies on 24.5-25.4 kg m 23 isopycnals in the eastern Luzon Strait exhibit a pronounced negative trend.…”

“…In recent years, salinity observations, especially the subsurface salinity observations, are made available from the Argo profiling floats [ Roemmich et al ., ]. The rapid growing of salinity data set has fueled many interests to resolve the subsurface salinity variability and its propagation in the North Pacific [e.g., Sasaki et al ., ; Ren and Riser , ; Li et al ., ; Yan et al ., ; Nagano et al ., ; Nan et al ., ; Nagano et al ., ]. For example, using the gridded product of temperature and salinity from Argo during the period of 2001–2008, Sasaki et al .…”

“…In recent years, salinity observations, especially the subsurface salinity observations, are made available from the Argo profiling floats [Roemmich et al, 2009]. The rapid growing of salinity data set has fueled many interests to resolve the subsurface salinity variability and its propagation in the North Pacific [e.g., Sasaki et al, 2010;Ren and Riser, 2010;Li et al, 2012;Yan et al, 2012Yan et al, , 2013Nagano et al, 2014;Nan et al, 2015;Nagano et al, 2016]. For example, using the gridded product of temperature and salinity from Argo during the period of 2001, Sasaki et al [2010 found the spiciness anomalies (salinity anomalies along isopycnals) generated and subducted at the surface of the northeastern subtropical North Pacific (1208W-1508W) and then propagated southwestward toward the western boundary and equatorial regions.…”

“…According to ventilated thermocline theory [ Luyten et al ., ; Liu and Pedlosky , ], the salinity anomalies advected by the mean circulation can theoretically reach the western Pacific by a direct interior pathway from subtropics to the tropics and affect the subsurface thermal variability here. Thus, as a first step toward ascertaining the fate of spiciness anomalies and its interior remote connections with other regions in the western Pacific, the author in the recent studies [ Yan et al ., ] conducted the analyses on the subsurface salinity variability downstream in the eastern Luzon Strait. The analyses yielded an important finding that the subsurface salinity variability in the eastern Luzon Strait exhibited a pronounced freshening trend; and this freshening is not a local‐forcing feature; instead, it is remotely connected to the surface salinity anomalies in the northwestern subtropical outcrop region (30°N‐35°N, 130°E–160°E), rather than that of the northeastern subtropical Pacific at 120°W–150°W.…”

A North‐South Contrast of Subsurface Salinity Anomalies in the Northwestern Pacific From 2002 to 2013

“…Ocean salinity observations, in particular subsurface salinity observations, are recently made available from Argo profiling floats (Roemmich & Steering, ). The growth of Argo observations in terms of spatial‐temporal coverage, combined with more than 50 years of repeating hydrographic observations along the 137°E section from the Japan Meteorological Agency, allows to resolve the subsurface salinity variability in the North Pacific (e.g., Li et al, ; Nagano et al 2015; Nan et al, ; Ren & Riser, ; Sasaki et al, ; Yan et al, , , ). For instance, based on Argo salinity from 2001 to 2008, Sasaki et al () found the salinity anomalies along the isopycnals were generated and subducted in the northeastern subtropical North Pacific (120–150°W), and these anomalies could then be advected southwestward to the western boundary and equatorial regions by the mean geostrophic current.…”